Suspension bottoming out - rear springs bad?

[gotchopsticks]

Hello all, 2002 GT, trying to figure out why my rear suspension seems to be bottoming out when I hit relatively mild bumps at 10-15 mph. By bumps I mean like the lip when the road transitions to a bridge and back to the road, or if there's a road repair that isn't perfectly level with the asphalt. Seems to be more pronounced when both wheels go over it vs just one wheel like in the case of driving over a manhole. I did notice that my pinion snubber is clearly impacting the diff, and since I am daily driving this car and not doing any hard launches or WOT acceleration, I can only assume the impact is happening during the aforementioned bumps. Front suspension does not bottom out going over the same bumps.

Rear suspension has Steeda Sport Springs, installed probably 15 years and 70k miles ago, just replaced the shocks with Koni str.t shocks, and running Nitto 555's 285/40/17 on 10.5" wheels; LCA, UCA, quad shocks, and sway bar are factory. I was planning to upgrade to aftermarket LCA's with poly bushings, but figured I might as well replace the springs while I was in there if those might be the culprit. As far as I can recall, the stance hasn't sagged from the fresh install, wheel wells still sit about 1.5" above the tire in front and rear, and the front springs still seem fine (also Steeda Sport springs) - just rebuilt the entire front suspension and it rides great. Any ideas on why the rear is bottoming out so easily, and what I can do to isolate & correct the issue? Thanks in advance!

 

[7991LXnSHO]

How many bodies are in the trunk?
Are the shocks working right, leaking or allowing excess bounce?

 

[gotchopsticks]

How many bodies are in the trunk?
Are the shocks working right, leaking or allowing excess bounce?

Ha! Even if there were a body in the trunk it would have to be a rather small body....

As far as I can tell the shocks are working correctly; I installed the brand new STR.T shocks about a week ago, followed the install guide and torqued them while the suspension was under load. If I try to make the rear bounce (open trunk lid and jump around on the rear), it does one rebound and then settles. I can't get it to bottom out by jumping on it, but I only weigh 160... I did notice that at rest, the rear springs are already pretty far compressed, I believe the top 4 coils are already touching and only 3 coils are still expanded. And as far as the snubber, its difficult to get a tape measure under there while at rest, but it appears that there is maybe 2" of travel before the axle would impact the snubber. Is that little amount of travel normal for these cars? I know it's lowered, but I guess I would expect a little more than 2" of travel for the rear given how much the shock can travel. Thanks.

 

[7991LXnSHO]

Then I would suspect it’s lowered enough to be the issue. I’d 1. check for clearance and see if shorter bump stops are an option, 1.5, make sure the rear bushings are all good. 2. Switch springs. My car came with light blue MM springs. It handled like a slot car and rode like a go cart. It scraped going in and out of some garages and almost flat driveways. B springs ride much better, scrape less and after getting used to some actual suspension travel, are much more enjoyable to drive.

 

[gotchopsticks]

Then I would suspect it’s lowered enough to be the issue. I’d 1. check for clearance and see if shorter bump stops are an option, 1.5, make sure the rear bushings are all good. 2. Switch springs. My car came with light blue MM springs. It handled like a slot car and rode like a go cart. It scraped going in and out of some garages and almost flat driveways. B springs ride much better, scrape less and after getting used to some actual suspension travel, are much more enjoyable to drive.

Yup that’s what I was thinking too. I was finally able to replicate the bottoming out by standing on the trunk opening with the lid up and really jumping up and down, and it seemed as though the noise/impact was coming from the center area under the trunk, so I’m pretty sure it’s not the shocks bottoming out. I’m also fairly sure the LCA & UCA bushing could use a replacement; 106k miles and 19 years old…I can see cracks in the rubber but I can’t see any significant tearing or obviously bad bushings, and the arms don’t have any noticeable play if I try to move them by hand. However, I do notice some weird movement when I’m going over a speed bump or driveway ridge at an angle; as my front goes down the other side of the bump/ridge and the rear driver tire hits the crest, the rear end seems to move sideways, and then even more so as the rear passenger tires goes over the crest. Definitely feels like there is movement in the rear that shouldn’t be happening.

I was planning to replace the LCA’s with MM HD’s or something comparable with poly on both ends, and I’ll try some FRPP B springs in the rear as well. I was thinking about replacing the UCA’s with OEM, but I can get decent non-adjustable UCA’s for a few bucks more….just not sure I want the NVH from the spherical bushings, and I know that having poly/poly on the UCA’s is a no-no for axle binding. For daily driver and no track time, think I’d be ok just doing the B springs, MM LCA’s, and OEM UCA’s? Any idea how the B springs pair with the Koni STR.T shocks? I can’t seem to find any definitive info on a google search for that combo of shock and spring. Thanks.

 

[7991LXnSHO]

@Warhorse Racing

 

[Warhorse Racing]

There are a lot of variables that factor in. I would suggest looking at photos of cars with Steeda Sport Springs to see if your car looks lower. LMR and AM should have photos on their sites. It sounds like the rear springs could be the reason your car is bottoming out.

Bushings can look okay, but be bad. If your LCA/UCA bushings are OEM, they are probably bad. You can use poly bushings in the UCA/LCA, but they should be 3-piece poly bushings. I've used Steeda UCAs/LCAs on my autocross cars for years and I've never had an issue with bind, or any damage to my torque boxes. They also come with a poly upper differential housing bushing. A spherical upper differential housing bushing is better for performance applications.

Your setup (staggered tires, stiffer shocks & struts, stock rear sway bar) is an "understeer" setup. This isn't as big of an issue for a daily driver, but, if you push the car to the limit, it will understeer.

The Ford Performance B Springs are progressive, so they will be more comfortable for daily driving. They will also raise the car up relative to the Steeda Sport Springs. With stock-style (not coil-over) springs, these cars handle better when they sit higher. The Steeda Sport Springs are as low as I would go. I use Ford C Springs (linear) on my solid rear axle cars. For performance driving, a linear rate would be more predictable.

 

[Mustang5L5]

Yup that’s what I was thinking too. I was finally able to replicate the bottoming out by standing on the trunk opening with the lid up and really jumping up and down, and it seemed as though the noise/impact was coming from the center area under the trunk, so I’m pretty sure it’s not the shocks bottoming out. I’

check your rear diff cover and see if it’s hitting your spare tire recess in the trunk pan. It’s common for this to contact with aftermarket covers on lowered cars.

 

[gotchopsticks]

check your rear diff cover and see if it’s hitting your spare tire recess in the trunk pan. It’s common for this to contact with aftermarket covers on lowered cars.

No visible impact to anything on the underside except for the pinion snubber. And it's more of a muffled 'thud' sound, doesn't sound like metal-metal contact.

Bushings can look okay, but be bad. If your LCA/UCA bushings are OEM, they are probably bad. You can use poly bushings in the UCA/LCA, but they should be 3-piece poly bushings. I've used Steeda UCAs/LCAs on my autocross cars for years and I've never had an issue with bind, or any damage to my torque boxes. They also come with a poly upper differential housing bushing. A spherical upper differential housing bushing is better for performance applications.

The Ford Performance B Springs are progressive, so they will be more comfortable for daily driving. They will also raise the car up relative to the Steeda Sport Springs. With stock-style (not coil-over) springs, these cars handle better when they sit higher. The Steeda Sport Springs are as low as I would go. I use Ford C Springs (linear) on my solid rear axle cars. For performance driving, a linear rate would be more predictable.

Understood, and thanks for the detailed response, and thanks to @7991LXnSHO for the info. I see that the Steeda, J&M, & MM HD LCA's have the 3-piece poly bushings while most others have normal poly bushings or spherical/rod ends, but I'd like to minimize NVH. Seems like the consensus across the Mustang forums is the MM (HD or XD) are the gold standard, Steeda is lesser used, J&M/UPR/BBK/Whiteline have mixed reviews, haven't seen much on BMR, QA1, or Team Z, but what I have seen seems pretty good...

So given that this is simply a daily driver, and I don't have any plans to take it to the track anymore (I have a twin-turbo Raptor as my weekend toy now), my primary goals are safety and comfort since 99% of the driving is city streets <55 mph on roads that could use some TLC. If I can get a some improvement over stock that would be nice, but I am by no means trying to turn this into a performance-focused vehicle, as I now live at 7,000+ elevation and the lack of O2 absolutely kills the performance on this N/A platform. I understand my comfort will be limited due being lowered, and I can accept that in lieu of having the 4x4 factory stance, but I don't want the comfort to get any worse in the name of increased performance.

I'm leaning towards doing the poly-poly LCA's (either MM, Steeda, or J&M), new rear springs, and poly pinion snubber as a first step, with the understanding that I will likely have to replace UCA's at some pint as well. I like to do troubleshooting and fixes in incremental phases so I can have a better idea of what the actual solution is, instead of replacing everything at once. Problem with the springs is that I'm having trouble finding just the rears. I did find the BMR, H&R (Super) Sport, and MM Road & Track springs that I can buy just the rears, but I'm not sure how that would pair with the Steeda springs up front, and Koni STR.T shocks. Like I said, the front is great, I just rebuilt everything (new inner/outer tie rods, control arms, ball joints, poly everything, Koni, MM CC plates, etc.) and it would be good to avoid having to rip it all apart to replace springs and do another alignment since the front springs seem to be just fine.

Sorry for the long-winded post, but given my use case do you think I'd run into any issues with the stated plan and parts?
LCA's: MM HD's, Steeda (p/n 555 4421), or J&M (SKU: 23859B) - all keeping springs perch and sway bar connection
Rear Springs: H&R (Super) Sport, or MM Road & Track https://www.maximummotorsports.com/MM-Road-Track-Spring-Kit-1996-98-HT-1999-04-GT-HT-P1094.aspx
Snubber: https://www.maximummotorsports.com/Pinion-Snubber-Urethane-1979-2004-solid-axle-P994.aspx

Thanks!

 

[1low03gt]

A little late to the party but….
your issue is the pinion snubber. I went threw the same thing years ago. Once I figured it out, I ordered a maximum motor sports snubber, and the bottoming out issue went away. That’s not saying you don’t have other issues, but what you described from coming from the centre. That is definetly the issue. The factory snubber is quite long as it hangs pretty low. Once you lower the stang lower then the norm this issue pokes its ugly head out. Now if your plans are for the H&R ss springs, it comes with a new snubber to accommodate the ride height. When I replaced my eibach sportlines for the H&R ss springs, it had the snubber in the box with the instructions. So don’t buy a separate one if you are going this route. Save the dollars lol.